Mizuno disclosed a combination locking device having removable U-shaped retaining rod in his U.S. Pat. No. 4,621,509 which however may have the following drawbacks:
1. For removing the retaining rod 3 from the cylindrical body case 10, the knob 16 is retracted along the long hole 15 formed in between the retaining rod 3 and the combination lock 1 when the number rings 22 are rotated to their opening combination so as to unlock the retaining rod 3 by disengaging the retaining piece 6 from the retaining grooves 5 formed on the retaining rod 3. The knob 16 should be always pulled sidewardly towards the combination lock 1 bearing a spring force acted by the spring 27, causing an inconvenience for opening the lock.
2. The knob 16 is positioned between the retaining rod 3 and the combination lock 1 so that a retraction operation for unlocking the retaining rod 3 is narrowly limited by the U-shaped rod 3 and the lock 1, causing an inconvenient unlocking operation on an ergonomic point of view.
3. In order to provide a reciprocative stroke for the movement of the knob 16, the cylindrical body case 10 should have an enough length to thereby prolong a total length of the case 10 and the lock 1, increasing a handling or storage volume therefore.
4. There is not provided with any device for changing or resetting a new unlocking combination.
Mayer et al disclosed a permutation lock in their U.S. Pat. No. 4,354,365. For changing a new combination, the closing bolt 18 should be retracted outwardly to retard an inwardly radial movement of the shank 31 of locking pin 28 so that the inner rings 36 are secured against rotation for re-setting a new combination. However, the retraction of the bolt 18 should be done very carefully. Otherwise, if the recess 32 between every two lands 76 of the bolt 18 is meeting the shank 31, the pin 28 will not be retarded radially outwardly, allowing a coupled rotation of the two rings 36, 38 thereby influencing the operation for changing the combination.
The present inventor has found the drawbacks of the conventional combination locks and invented the present combination lock which can be rotatably unlocked quite conveniently.